Non-volatile memory devices retain their stored data even when their power supplies are interrupted. Flash memory devices are well known as representative non-volatile memory devices. A flash memory device includes a unit cell with an electrically isolated charge storing element. A threshold voltage of a flash memory cell can be regulated by storing charges in the charge storing element or ejecting charges from the charge storing element, so as to store predetermined logical data in the flash memory cell and allow the stored logical data to be read therefrom. A flash memory device can write and/or erase data electrically.
Conventionally, a flash memory device requires low operation voltages (e.g. a program voltage, an erase voltage and/or a verify voltage, etc.). With the rise of operation voltages such as a program voltage and/or an erase voltage, characteristics of an oxide layer that is formed to surround a charge storing element may become degraded and result in erroneous operation, such as loss of the data stored in the charge storing element.
In addition, flash memory devices are required to have long-term data retention characteristics. However, charges stored in the charge storing element may leak through an oxide layer (e.g., an oxide layer interposed between the charge storing element and a semiconductor substrate). Thus, stored data may be lost over time and thereby cause malfunction of the flash memory device.